1. Field of the Invention
The present invention relates to a power amplifier and more particularly to a microwave power amplifier topology that functions as a low-noise amplifier when the input signal level is low and automatically switches to high-power amplification for relatively high input signal levels.
2. Description of the Prior Art
Radio frequency and microwave communication systems are known to place ever-increasing demands on the linearity and efficiency of power amplifiers. Unfortunately, conventional power amplifiers operate at maximum efficiency at or near saturation. Thus, in order to accommodate communication signals having varying amplitudes, systems utilizing conventional power amplifiers normally operate at less than peak efficiency for a substantial portion of the time.
In order to solve this problem, so-called Doherty amplifiers have been developed. Doherty amplifiers were first introduced by an inventor having the same name and described in; xe2x80x9cRadio Engineering Handbookxe2x80x9d 5th edition, McGraw Hill Book Company, 1959, pp. 18-39, as well as U.S. Pat. No. 2,210,028, hereby incorporated by reference. The standard topology for a Doherty amplifier includes a carrier amplifier, operated in a Class AB mode, and peak amplifier, operated in a Class C mode. A quadrature Lange coupler is used at the input so that the carrier amplifier and peak amplifier signals will combine in phase. A quarter wave amplifier is provided at the outputs of the amplifier. In such amplifiers, as the input RF drive signal to the carrier amplifier increases, the carrier amplifier is driven to the point of saturation for maximum linear efficiency. The peak amplifier is used to maintain the linearity of the output signal when the carrier amplifier begins to saturate.
Such Doherty amplifiers have been known to be used in various microwave and RF applications. Examples of such applications are disclosed in U.S. Pat. Nos. 5,420,541; 5,880,633; 5,886,575, 6,097,252 and 6,133,788. Examples of such Doherty amplifiers are also disclosed in xe2x80x9cA Fully Integrated Ku-Band Doherty Amplifier MMIC,xe2x80x9d by C. F. Campbell, IEEE Microwave and Guided Wave Letters, Vol. 9, No. 3, March 1999, pp. 114-116; xe2x80x9cAn 18-21 GHz InP DHBT Linear Microwave Doherty Amplifierxe2x80x9d, by Kobayashi, et al, 2000 IEEE Radio Frequency Integrated Circuits Symposium Digest of Papers, pages 179-182, xe2x80x9cA CW 4 Ka-Band Power Amplifier Utilizing MMIC Multichip Technology,xe2x80x9d Matsunaga, et al., 1999, GaAs IC Symposium Digest, Monterey, Calif., pp. 153-156, all hereby incorporated by reference.
The systems mentioned above are known to provide relatively good phase linearity and high efficiency since the power amplifier only needs to respond to constant or near constant RF amplitude envelopes. Unfortunately, the RF amplitude envelopes of multi-carrier signals (multi-frequency signals), used for example in satellite communications systems, change with time as a function of the bandwidth thus exhibiting noise-like characteristics. Power amplifiers utilized in multi-carrier systems must be able to operate over a relatively large instantaneous bandwidth while providing relatively good phase linearity for RF signals having non-constant envelopes.
In addition, such power amplifiers, used as low-noise amplifiers (LNA), for example, as a first amplification stage in an RF or microwave receiver, must be able to provide linear amplification at a relatively high efficiency. Unfortunately, in applications when the RF drive signal has a non-constant RF envelope, for example, as in a multi-carrier satellite communication system, the Doherty amplifier is operated such that the carrier amplifier is operated at about one half of its maximum power capability during low power operation to provide relatively low noise performance. Such operation results in relatively low linearity and low efficiency.
In order to increase the linearity and efficiency of Doherty amplifiers used in applications of non-constant RF envelopes, various techniques have been used. For example, U.S. Pat. No. 5,739,723 discloses an active bias circuit which biases the peaking amplifier to improve the efficiency. Unfortunately, the active bias circuit includes a number of resistors which increases the overall power consumption of the circuit thus providing a less than optimum efficiency.
In order to minimize the bias power consumption of such a Doherty amplifier, U.S. Pat. No. 5,568,086 discloses a combining network for combining the output signals of the carrier amplifier and the peak amplifier to provide improved impedance matching. The combining network includes a pair of quarter waves transformers and a number of quarter wave phase shifting elements. Unfortunately, efficiencies of only 40%-50% were realized in the combining network disclosed in the ""086 patent. Power efficiency in many applications such as satellite communication systems is a critical factor. Thus there is an ever-increasing need to further improve the efficiency of power amplifiers used in such applications.
The present invention relates to a microwave amplifier and more particularly to a microwave amplifier configured as a Doherty amplifier. The amplifier includes a carrier amplifier, a peak amplifier, a Lange coupler at the input of the amplifiers and quarter wave amplifier at the output of the amplifiers. In order to further increase the efficiency, the Doherty amplifier is formed from HEMT/HBT technology to take advantage of the low-noise performance of HEMTs and the high-linearity of HBTs to form a relatively efficient amplifier that functions as a low-noise amplifier at low power levels and automatically switches to high-power amplification for relatively high-impact RF power levels.